The present invention relates to structures and methods for protecting inert anodes and other electrodes and electrode support materials from degradation by a cryolite-based molten electrolyte bath, and from HF/O2 and other gases generated in an electrolytic cell. The present invention also improves metal production, such as aluminum production, by limiting bath and metal contamination and reducing thermal shock during initial preheating and placement of anodes in electrolytic cells.
Aluminum is produced conventionally by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures between about 850xc2x0 C. and 1000xc2x0 C.; the process is known as the Hall-Heroult process. This process is well known and described for example in U.S. Patent Specification No. 5,279,715 (La Camera et al.) A Hall-Heroult reduction cell typically comprises a steel shell having an insulating lining of refractory material, which in turn has a lining of carbon that contacts the molten constituents. The electrolyte is based on molten cryolite (Na3AlF6) which may contain a variety of additives such as LiF, CaF2, MgF2 or AlF3, and contains dissolved high purity alumina (Al2O3). The carbon lining has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminum as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks. In addition, the penetration of sodium species, other substances contained in cryolite, or air leads to the formation of toxic compounds including cyanides. Anodes are at least partially submerged in the bath and are subject to the same conditions.
The Hall process, although commercial today, has certain limitations, such as the requirement that the process operate at relatively high temperatures, typically around 970xc2x0 C. to 1000xc2x0 C. The high cell temperatures are necessary to achieve a high alumina solubility. At these temperatures, the electrolyte and molten aluminum progressively react with most carbon or ceramic materials, creating problems of electrode erosion, which can cause cell contamination and metal and electrolyte containment. Thus, it is generally thought that the electrolyte constituents are adverse to the rest of the cell.
Electrolytic reduction cells must be heated from room temperature to approximately the desired 1000xc2x0 C. operating temperature before the productions of metal can be initiated. Heating should be done gradually and evenly to avoid thermal shock to the cell components which can in turn cause breakage or spalling. The heating operation minimizes thermal shock to the lining, the electrodes and other attached structural assemblies upon introduction of the electrolyte and molten metal to the cell. Prior art carbon anodes can be placed into the electrolyte at ambient temperature, and heated by the energy of the cell to operating temperatures, at which time the nominal current of the anode will be attained.
Newer, ceramic inert anodes have much longer lives, but both the anodes and their supports are prone to thermal shock and therefore generally need to be preheated in a furnace or the like outside of the electrolytic cell prior to insertion into the hot electrolyte. The thermal shock/cracking can occur both during movement of the anodes into position and during their placement into the molten salt. Thermal shock relates to the thermal gradient (positive or negative) through the anode that occurs during the movement from the preheat furnace to the cell, and also upon insertion of the anodes into the molten salt. A thermal gradient as low as 50xc2x0 C. can cause cracking.
A variety of attempts have been made to introduce various particulates into the inert anode or to cover them with various protective materials, but it is virtually impossible to prevent some dissolution, and eventually such attempts lead to a certain amount of contamination of the bath and aluminum being produced. In one attempt to protect electrodes in an electrolysis cell from thermal shock during start-up, U.S. Patent Specification No. 4,265,717 (Wiltzius), taught protection of hollow cylindrical TiB2 cathodes by inserting aluminum alloy plugs into the cathode cavity and further protecting the cathode with a heat dispersing metal jacket having an inside heat insulating layer contacting the TiB2. There, the heat insulating layer was made of expanded, fibrous kaolin-china clay (Al2O3 2SiO2 2H2O), which would subsequently dissolve in the molten electrolyte, introducing Si. A refractory repair mass is taught in U.S. Patent Specification No. 5,928,717 (Cherico et al.). There, a powder mixture of alumina, metallic combustible such as magnesium, zirconium, chromium and aluminum plus additive selected from aluminum fluoride, barium sulfate, cerium oxide or calcium fluoride are used with an oxygen stream, under pressure, to contact and cure non-uniform crystalline structures and the like at the surface of used refractory. This however, primarily relates to repair and to already present refractories which have been contacted with molten aluminum or molten glass.
In the design of inert anodes for aluminum or other metals production, an array or assembly of uncovered inert anodes can be mounted on a cast refractory insulating lid below a metal plate, through which a continuous electrical path from the cell is provided. In this arrangement, shown in FIG. 3 of U.S. Patent Specification Nos. 6,551,489 B2 and 6,558,526 B2 (both D""Astolfo Jr. et al.), it is necessary to provide protection of the metal plate and cast refractory. The problem, however, is that most refractory materials are not able to withstand the severe thermal shock and gradients encountered during preheat operations without cracking or to withstand a certain amount of dissolution during cell operation. This design is costly and requires a major amount of assembly.
Aluminum electrolysis cells have historically employed carbon anodes on a commercial scale. The energy consumption and cost of aluminum smelting can be significantly reduced with the use of inert, non-consumable, and dimensionally stable anodes. Use of inert anodes rather than traditional carbon anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also realized because inert anodes produce essentially no CO2 or CF4 emissions.
Inert anodes can be made of, for example a ceramic, metal ceramic xe2x80x9ccermetxe2x80x9d or metal containing material. Some examples of ceramic inert anode compositions are provided in U.S. Patent Specification Nos. 6,126,799; 6,217,739 B1; 6,372,119 B1; and 6,423,195 B1 (all Ray et al. respectively), herein incorporated by reference. These anodes comprise a ceramic phase and may also comprise a metal phase. They are essentially void free and while they exhibit low solubility and good dimensional stability there is still some corrosion in Hall cell baths at 1000xc2x0 C.
In addition to electrode thermal shock problems and electrode support and other cell erosion and contamination problems, an improved, simplified and more cost effective overall design of the electrode/electrode support is needed.
It is one of the main objects of this invention to protect inert cermet anode electrodes and attached assemblies from thermal shock and chemical reactants. It is another main object of the invention to provide a simplified electrode assembly which contains a minimal of materials, parts and contaminants. These and other objects are accomplished by providing an electrolysis apparatus comprising a plurality of anodes, each anode having a lower portion immersed in molten electrolyte bath, wherein a solid material selected from the group consisting of alumina and cryolite, and mixtures thereof, together with a minor effective amount, about 5 wt. % to 25 wt. % of cementitious binder, said solid material contacting and circumscribing at least an upper portion of at least one of said anodes. The solid material can be applied by molding/casting, dipping, spraying or the like, and can be made that upon dissolution on or very little impurities are introduced into the molten bath.
The invention also provides an electrolysis apparatus comprising an inert anode system comprising at least one inert anode having a lower portion in contact with a molten salt bath, where at least an upper portion of the inert anode contacts and is circumscribed by a solid material subject to attack by gases from the bath, wherein the solid material is selected from the group consisting of alumina-cement and cryolite-alumina, both of which will dissolve in the presence of the molten salt bath. The alumina-cement material is preferably at least 92% pure Al2O3, insulating and very advantageously, highly temperature resistant. The alumina-cryolite material is preferably about 40 wt. % to 80 wt. % cryolite, at least 2 wt. % alumina and 5 wt. % to 25 wt. % of a high temperature resistant cementitious material. By xe2x80x9ccryolitexe2x80x9d is meant, sodium aluminum fluoride which may contain various alkali and alkaline earth elements, such as calcium, magnesium, potassium, lithium, and beryllium in various ratios as well as the specific formula Na3AlF6. Alumina can also be used, as a major component with from 5 wt. % to 15 wt. % heat resistant refractory cementitious material. The alumina-cement structure can, advantageously be formulated to be 50 vol. % to 95 vol. % dense (that is having 5 vol. % to 50 vol. % porosity) allowing air inclusions providing advantages of over 1000xc2x0 C. preheating before insertion into the bath. The alumina can also contain up to 15 wt. % other oxides, such as, for example, CaO2, SiO2 and others as well as the cement previously mentioned.